The invention relates generally to processes for hot forming materials, and particularly to a process for manufacturing powder metallurgy (P/M) preforms which are preconditioned and characterized for further conditioning to achieve microstructural refinement by processes such as dynamic recrystallization within the material comprising the preform.
Modern high strength and high temperature materials such as rapid solidification technology (RST) alloys have unique and complex composition, microstructure and mechanical properties, and are characterized by good oxidation resistance, low density and other favorable properties for high temperature applications. However, powder materials and materials produced by advanced ingot metallurgy are in general characterized by poor hot workability. Using the dynamic materials modeling approach of the cross reference, behavior of a material during forging, extrusion, rolling, sheet metal forming, or other hot forming material processes may be predicted and characterized by defining in advance response of the material to demands of the process. Optimum hot workability and precise microstructural control may therefore be achieved in the material. The teachings of the cross reference and background material disclosed therein are incorporated herein by reference.
According to the cross reference optimum processing parameters are selected from energy dissipation maps generated from inelastic constitutive equation data which describe flow behavior of a workpiece material as a function of temperature, strain, and strain rate, and which describe power dissipation efficiencies for the material favoring metallurgical processes that enhance both hot workability and mechanical properties in the material. Temperature and strain rates selected from an energy dissipation map are determinative of the optimum rate at which energy may be applied to hot form the material through desirable metallurgical processes without fracture, to ameliorate effects of pre-existing defects in the material, and to obtain a microstructure consistent with prescribed engineering properties for the product.
The invention defines a process for manufacturing P/M preforms which are conditioned for optimum intrinsic workability through improved interparticle bond strength of a P/M compact and through avoidance of certain defects in the product. A major limitation of existing RST methods for manufacturing alloy powders is the inability to process powders into billets which are free of prior particle boundary and other defects and which have a microstructure providing enhanced workability. The invention overcomes this limitation in a process for producing substantially defect free P/M billets having predetermined microstructure and mechanical properties for subsequent near net shape hot forming without prior trial-and-error determinations of process parameters.
The process of the invention comprises steps of first preconditioning P/M powder by either degassing the powder under vacuum in a can at predetermined temperature and blind compacting the canned powder at a temperature of about 0.75 melting point (.degree.K.) of the powder; or cold compacting the powder to about 75% density, degassing it at suitable temperature and vacuum hot pressing it at about 0.75 melting point. The compact is characterized by developing a processing map to select temperature and strain rate for the preconditioned compact at which dynamic crystallization is achieved in the billet. The compact is extruded at the selected temperature and corresponding strain rate using a streamlined die configured according to process modeling techniques.
It is a principal object of the invention to provide a process for consolidating P/M preforms under optimum working conditions.
It is a further object of the invention to provide a process of preconditioning a P/M preform to optimize hot workability of the material comprising the preform.
It is yet another object of the invention to provide a process for preconditioning P/M preform billets and optimizing hot forming process parameters to achieve microstructure refinement in the billets to produce near net shape products.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.